But the most significant aspect of the bombing was not its aftermath but its implication: the sectarian war unfolding in Syria is beginning to engulf Lebanon as well. We don’t yet know who carried out the attack, which took place in the largely Shiite neighborhood of Bir Hassan. The Iranian Ambassador, who survived, blamed the Israelis. A jihadi group known as the Abdullah Azaam Brigade claimed responsibility, according to a Lebanese press report. A leader of Hezbollah, the Lebanese armed group, blamed takfiris—Muslims who accuse other Muslims of apostasy.

The best bet is that the attack was carried out by a group close to the Syrian rebels, who are trying to topple the government of Bashar al-Assad. In the two and a half years since the rebellion began, the war in Syria has become an almost entirely sectarian struggle. On one side sits the Assad government, dominated by Alawites, who consider themselves Shiites. Arrayed against them (and Syria’s other minorities) is the vast majority of Syria’s population, which is Sunni Muslim. It’s a death match; the Alawites rightly fear that if they lose the war they will probably vanish from Syria altogether.

Only a year ago, Assad’s government appeared to be on the verge of collapse. The rebels were gaining ground across the country. The Sunni-led states in the region, including Saudi Arabia and Turkey, were funnelling arms to the rebels. And then, at the last possible moment, the Iranian regime launched a dramatic rescue mission. The number of Iranian cargo planes flying into Damascus jumped steeply, carrying guns, ammunition, and, crucially, men. Hundreds, possibly even thousands, of members of the Quds Force, the external branch of the Iranian Revolutionary Guard, fanned out across Syria, offering battlefield advice and helping to monitor rebel communications. Quds Force officers set up a command post in Damascus and largely took over the direction of the war. (I wrote about the role of the Quds Force in Syria in a recent piece.) The Iranian regime, despite the severe economic pain being inflicted on them by Western sanctions, extended the Assad regime a loan of seven billion dollars. At the same time, the Iranians called on their protégés next door, in Lebanon: as many as two thousand Hezbollah fighters crossed the border to engage in direct combat with the rebels. For the Iranians, the motivation was simple: they have spent thirty years trying to insure that they are surrounded by friends. “If we lose Syria,” one Iranian cleric said earlier this year, “we cannot keep Tehran.”

The Iranian intervention proved decisive. The Assad regime has regained the initiative. It is no longer losing territory; it still controls Damascus, most of Aleppo, and the highways leading to the Alawite strongholds on the Mediterranean coast. For the Iranians, this has come at a cost: at least seven Iranian officers have been killed in Syria, including two generals. One of them, Hassan Shateri, had been in charge of Quds Force operations in Lebanon. Indeed, according to Western security officials, most of the Iranians in the embassy in Beirut are members of the Quds Force.

Lebanon has always been an arena for confrontations between the region’s more powerful regimes. The Lebanese civil war, which ended in 1990, after fifteen years, pulled in the Syrian, Israeli, and even American militaries. (It’s no small irony that the suicide attacks on the American Embassy and Marine barracks in Beirut in 1983, which killed more than two hundred and fifty people, were orchestrated with Iranian help.) In more ways than one, Lebanon is the Middle East’s playground.

Since Lebanon’s civil war ended, the peace has held because the country’s main confessional groups—the Sunnis, the Shiites, and the Christians—have been very careful not to grab too much power. It’s a delicate balance, and now the war in Syria is overwhelming it. When Hassan Nasrallah, the leader of Hezbollah—the vanguard of the Shiites—looks to the east, he sees an ocean of angry Sunnis. Without Assad in Damascus—the critical link in the Iranian supply line—Hezbollah could be doomed. Hence Nasrallah’s decision to send his fighters across the border. The day before the Embassy bombing, Nasrallah reaffirmed his commitment to Assad. “As long as there are reasons for us to stay in Syria, we will stay there,” he said. For months, Syrian rebels have been vowing to take revenge on Hezbollah and their Iranian benefactors. It’s a good bet that on Tuesday they did.

The longer the war in Syria goes on, the stronger the sectarian calculus in Lebanon will become. Last year, I spent a couple of days tagging along with a mid-level Hezbollah fighter. The fighter, who went by the name of Dani, refused to acknowledge that Hezbollah was sending fighters into Syria, even though it was an open secret. But he was unashamed to show his support for Assad, or to contemplate the consequences that support might have for his own country. “If the Sunnis take over Syria,” Dani said, “we’re going to be fighting them in Beirut.”